Currently, it works for Bills in Congress (back to the 104th Congress), U.S. Public Laws (ditto), the U.S. Code, the Federal Register (to 1995 for text, 1994 for tables of contents), and as of today, the Code of Federal Regulations.

If they're free sources, and the sites are easily searchable, why is an URL shortener necessary? Well, these sites, thankfully, are loaded with other features that make it difficult for them to offer clean, simple, and short URLs to access legal text.

So why has no one done this before? Honestly, I have no idea. Is it really necessary? Unless you're Rain Man, I think it can't hurt, especially if you want to send or show people links to primary (or close-to-primary) legal source material.

My main idea was to let people use L4w.us links as easy-to-remember shortcuts to public versions of our laws, so instead of going to the destination sites and searching, or trying to remember the multiple URL schemes, if you know what you're looking for (sorry, no search yet), just type it in and go.

I also put it together to help simplify ontolawgy™ LLC's bulk data downloading programs. The overhead on the server is minimal, so I thought I'd share. Feel free to use it, and tell people how it's going; use L4w.us links in blog posts, articles, theses, student projects, non-profit citizen-engagement applications/campaigns, etc. Just don't bake it into a commercial software or hardware (e.g., a book) application without permission.

Currently, the site doesn't offer any fancy features, and we don't plan to. But it also doesn't track you. We maintain basic access logs with IP addresses to make sure people aren't abusing the service (e.g., DDOS-ing it for some unknown reason or incorporating the service into commercial activity without a license), but other than that, whatever laws you look at are between you, the site providing the content, and whoever is monitoring your internet traffic.
Suggestions for how to improve the site which resources to add are most welcome here in the comments, on Twitter, or by email.

*If you haven't picked up on it by now, technically, L4w.us is not really an URL-shortener, it's more like an API for people to access other (simple) people-facing APIs, but without scaring them away by calling it an API.